1. Field of the Invention
The present invention relates to a motor-operated flow control valve suitable for use in internal combustion engines, and more particularly to an exhaust gas recirculation control valve for internal combustion engines.
2. Description of the Related Art
Conventional motor-operated flow control valves have such a known structure that a rotor unit of a motor for driving a valve is rotatably supported by a pair of ball bearings disposed in upper and lower portions of the rotor unit.
Those conventional motor-operated flow control valves are disclosed in, for example, U.S. Pat. Nos. 4,432,318, 4,381,747, 4,378,767, 4,378,768, 4,414,942, 4,397,275 and 5,184,593, JP-A-7-190227 and 7-190226, etc.
In the conventional motor-operated flow control valves, because the rotor unit of the motor is rotatably supported by two ball bearings disposed in upper and lower portions of the rotor unit, there inevitably occurs relative wobbling between inner and outer races of each of the ball bearings. When used in internal combustion engines, therefore, such a motor-operated flow control valve tends to resonate with rotative vibration of the internal combustion engine, resulting in a problem that the useful life of the valve itself and a device including the valve is shortened.
To lessen the relative wobbling between the inner and outer races, there is also known a structure that the rotor unit is supported by two bearings under a state where a preload is applied to press the rotor unit in one direction. Specifically, for example, an outer race of one ball bearing is supported by a rigid body such as a housing, and an outer race of the other ball bearing is pressed by a spring such as a spring washer or a coil spring. With such a structure, however, because the preload generated by the spring washer or the like is applied to balls of the ball bearing as well, frictional torque occurred upon starting the rotor unit to rotate is increased. This results in another problem that the motor is required to produce a larger torque at the start-up.
An object of the present invention is to provide a motor-operated flow control valve for internal combustion engines which is less affected by vibration and has a longer useful life.
Another object of the present invention is to provide a motor-operated flow control valve for internal combustion engines which does not require a motor to produce a larger torque at the start-up.
To achieve the above objects, according to the present invention, in a motor-operated flow control valve comprising a rotor shaft reciprocating with rotating motion of a motor, and a valve head movable to open and close an orifice with the reciprocating motion of the rotor shaft, specific frequency of a rotor unit of the motor is set to be higher than the secondary vibration frequency of rotation of a 4-cycle internal combustion engine. With this feature, when applied to any of internal combustion engines having four, six and eight cylinders, the motor-operated flow control valve will not give rise to a resonance phenomenon and therefore has a longer useful life.
In the above motor-operated flow control valve, preferably, the rotor unit comprises an integral magnet, a single ball bearing and a resin-made magnet holder for supporting the magnet and the ball bearing, the magnet, the ball bearing and the magnet holder being formed into an integral structure. With this feature, the weight of the rotor unit can be so reduced as to make the specific frequency of the rotor unit have a value not resonating with engine vibration.
Further, to solve the above objects, according to the present invention, in a motor-operated flow control valve comprising a rotor shaft reciprocating with rotating motion of a motor, and a valve head movable to open and close an orifice with the reciprocating motion of the rotor shaft, a rotor unit of the motor comprises an integral magnet, a single ball bearing and a magnet holder for supporting the magnet and the ball bearing, the ball bearing having an outer race held fixed under a preload. With this feature, frictional torque occurred upon starting the rotor unit to rotate is reduced and torque required for the motor to produce at the start-up is made smaller.